Enhanced capacity of downlink lower layer (layer 1 or L1/layer 2 or L2) control signaling is currently considered in 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) for meeting higher control signaling loads in new deployment scenarios as well as for introducing L1/L2 control signaling that can be demodulated by using UE (user equipment) specific reference symbols. One such deployment scenario which would benefit from an enhanced L1/L2 control signaling design is heterogeneous networks with single cell identities. Another deployment scenario where enhanced L1/L2 control signaling could be of interest is for carrier aggregation with extension carriers, i.e. carriers that are not backward compatible. In release 10 of LTE, a new L1/L2 control signaling design was specified for wireless backhaul communications between a donor eNB (evolved node B) and relay node (RN). A similar L1/L2 control signaling design is considered to be introduced also on the access link, i.e. the link between an eNB and a UE, as a tool for enhancing the L1/L2 control signaling capacity and open up for demodulation of L1/L2 control signaling using UE specific reference symbols.
Heterogeneous networks are characterized by deployments with a mixture of cells of differently sized and overlapping coverage areas. One example of such network 100 is where pico cells 108 are deployed within the coverage area of a macro cell 106, as illustrated in FIG. 1 which shows different user equipment UE 102 in the cells 106, 108 associated with different base stations 104. A pico cell 108 is a small cellular base station 104 transmitting with low output power and typically covers a much smaller geographical area than a macro base station 104. The small cellular base station 104 may be referred to as a low power node, whereas a macro base station 104 represents a high power node. Other examples of low power nodes in heterogeneous networks are home base stations and relays.
Heterogeneous networks represent an alternative to densification of macro networks, and have classically been considered and motivated in cellular networks with areas of non-uniform user distributions, i.e. geographical areas with typical clustered traffic hotspots. There small cells covering the traffic hotspot can off-load the macro cell and thus improve both capacity and the overall data throughput within the coverage area of the macro cell. In emerging mobile broadband applications, there is however a continuous demand for higher bit rates and therefore it is of interest to deploy low power nodes not necessarily to cover traffic hotspots only but also at locations within the macro cell coverage where the signal-to-noise ratio prevents high data bit rates.
The traditional way of deploying cellular networks is to let different base stations 104 form separate cells 106, 108, as illustrated in FIG. 1, in which each cell 106, 108 has its own cell identity (cell-id). This means that physical layer signals transmitted from a base station 104, as well as signals received by a base station 104, are associated with a cell-id that is different from the cell identities used by neighbor base stations 104. Typically, a base station 104 in a cellular system 100 transmits its own unique signals for broadcasting of cell information and for cell synchronization. In LTE (Long Term Evolution), base stations 104 transmit cell-specific reference signals and downlink scrambling is applied to transport channels and L1/L2 control signaling where the scrambling sequence depends on the cell-id in order to randomize inter-cell interference. The use of different cell identities forms the base for reusing the same physical layer resources within a certain coverage area. For example, resources used in the macro cell 106 can also be used by the pico cells 108 in FIG. 1. The benefits of reusing resources within a geographical area are sometimes referred to as cell splitting gains. A challenge though with this type of deployment is mitigation of inter-cell interference between macro cells 106 and pico cells 108, in particular interference from the high power macro node towards the pico cells 108.
An alternative to the traditional way of deploying heterogeneous networks is to let low power nodes (related cell is denoted with reference numeral 204) within the macro coverage use the same cell-id as the macro cell 202 as illustrated in network 200 in FIG. 2. This deployment scenario is sometimes referred to as heterogeneous networks with single cell identity, in which base station nodes 104 in the network 200 are often referred to as transmission/reception points, or simply points.
Thus, UEs 102 within the geographical area defined by the coverage of the high power macro point will be served with signals from points associated with the same cell-id. Other neighbor macro points will typically use different cell identities. The concept of points is closely related with techniques for coordinated multipoint (CoMP) transmissions and receptions. In this context, a point corresponds to a set of antennas covering essentially the same geographical area in a similar manner. Antennas correspond to different points when they are sufficiently geographically separated and/or having antenna diagrams pointing in sufficiently different directions. Techniques for CoMP entail introducing dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where a point from a scheduling point of view is operated more or less independently from the other points.
Characteristics for heterogeneous networks 200 with single cell id are the need for tight coordination of the transmissions across points within the coverage defined by the macro point and that received signals at the UE 102 appear coming from a single cell 202. A fundamental difference from deployments with multiple cell identities, as for instance the one illustrated in FIG. 1, is the avoidance of inter-cell interference across points within the coverage defined by the high power macro point. However, in contrast to multiple cell identity approach, the single cell identity approach requires both fast connections (such as fiber) and tight transmission coordination between the macro point and the pico points. Which physical signals and channels that are sent from a certain point, or points, can be deployment specific but broadcast and control channels may all be transmitted from the high power point only while data can be transmitted to a UE 102 also from low power points by using shared data transmissions relying on UE specific reference symbols. One example would be a base station 104 serving one or more sectors on a macro level as well as having fast fiber connections to remote radio units (RRUs) playing the role of the other points sharing the same cell-id. Those RRUs could represent low power points with one or more antennas each. Another example is when all the points have a similar power class with no single point having more significance in than the others. The base station 104 would then handle the signals from all RRUs in a similar manner.
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform spread OFDM (DFT-spread OFDM) in the uplink. In OFDM transmissions, a set of modulated symbols is transmitted over narrowband and orthogonal subcarriers, where the number of subcarriers defines the transmission bandwidth of the OFDM signal. In DFT-spread OFDM, the set of modulated symbols is first pre-coded before generating the OFDM signal, where the pre-coding aims to provide power characteristics of the OFDM signal suitable for transmit power limited terminals.
A basic LTE physical resource can thus be seen as a time-frequency grid 300 as illustrated in FIG. 3, where each resource element 302 corresponds to one subcarrier during one OFDM symbol interval. In LTE, frequency spacing between subcarriers is 15 kHz. Time domain is plotted as reference numeral 304, whereas frequency domain is plotted as reference numeral 306.
In the time domain 304, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes of 1 ms. A subframe is divided into two slots, each of 0.5 ms time duration. Each slot comprises of either 6 or 7 OFDM symbols depending on the selected cyclic prefix length. LTE supports two cyclic prefix lengths, commonly referred to as the normal and extended cyclic prefix, respectively. The cyclic prefix, inserted in the beginning of the OFDM symbol interval, aims to mitigate inter-symbol interference.
The resource allocation of data in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain 304 and 12 contiguous subcarriers in the frequency domain 306. Two in time consecutive resource blocks represent a resource block pair and corresponds to the time interval upon which scheduling operates. A user can be assigned data in one or multiple resource block pairs. Transmissions in LTE are dynamically scheduled in each subframe where the base station 104 transmits assignments and/or grants to certain user equipments 102 via the Physical Downlink Control Channel (PDCCH). The PDCCH is transmitted in the first OFDM symbol(s) in each subframe and spans over the whole system bandwidth. A UE 102 that has decoded downlink control information, carried by a PDCCH, knows which resource blocks in the subframe contain data aimed for the user equipment 102. In LTE, data is carried by the physical downlink shared channel (PDSCH).
FIG. 4 illustrates an example for LTE data transmission 400 in the form of a DL subframe 402 having a control region 404 and a data region 406. Individual blocks include cell specific reference symbols 408, control blocks 410 and data blocks 412.
Demodulation of sent data requires estimation of the radio channel which is done by using transmitted reference symbols 408, i.e. symbols known by the receiver. In LTE, cell specific reference symbols 408 are transmitted in all downlink subframes and in addition to assist downlink channel estimation they are also used for mobility measurements performed by the user equipments 102. LTE supports also UE specific reference symbols 408 aimed only for assisting channel estimation for demodulation purposes. The UE specific reference symbols 408 are transmitted in the data region 406 such that they do not collide with the cell specific reference symbols 408.
The length of the control region 404, which can vary on subframe basis, is conveyed in the Physical Control Format Indicator CHannel (PCFICH). The PCFICH is transmitted within control region 404, at locations known by terminals. After a terminal has decoded the PCFICH, it thus knows the size of the control region 404 and in which OFDM symbol the data transmission starts. Also transmitted in the control region 404 is the Physical Hybrid-ARQ Indicator Channel. This channel carries ACK/NACK responses to a UE 102 to inform if the uplink data transmission in a previous subframe was successfully decoded by the base station 104 or not.
Downlink assignments and uplink grants are conveyed in Downlink Control Information (DCI) messages carried by PDCCHs. Multiple DCI formats of different payloads are supported in LTE and reflect the different transmission modes that UEs 102 can be configured to operate within. Encoded into the DCI message is a specific radio network temporary identity (RNTI), used to either address a single user or a group of users, or all users connected to the cell. Thus, LTE supports multiple types of RNTIs considered for different purposes such as unicast transmissions and transmissions of system information, paging and random access responses. In the case of unicast data transmissions, a UE specific RNTI is encoded into the DCI message. UEs 102 monitor PDCCH transmissions and check if it's unique RNTI match the received DCI message. If it does, it demodulates the message and receive/transmit data in accordance with the control message. Which DCI format that is used in the transmission is unknown to the UE 102 and it therefore needs to blindly decode PDCCHs under different DCI format hypotheses.
In order to facilitate link adaptation on PDCCH transmissions, as a request to meet different radio reception conditions, LTE has introduced a certain mapping structure of PDCCH to resource elements in which 36 resource elements are grouped into Control Channel Elements (CCEs). The PDCCH can then be mapped to 1, 2, 4 or 8 CCEs depending on the DCI payload and the desired coding rate of the control information. Thus, the number of resource elements used for a PDCCH transmission is 36n, where n=1, 2, 4, 8. Depending on the coding rate the robustness of the signals can be modified in view of the transmission quality.
Typically, multiple PDCCHs are transmitted within the control region as a request of addressing multiple users in the same subframe. Exactly where in the control region the PDCCH is transmitted to a certain user is not known in advance, implying that the UE 102 needs to search blindly for the locations of the PDCCHs. However, in order to reduce the search burden of blind detection attempts of possible PDCCH locations UE specific search spaces and common search spaces can be defined.
The UE specific search space refers to limit the possible set of PDCCH resources that can be allocated for a particular UE 102 to address unicast transmissions. Each connected UE 102 in the cell is configured with its own search spaces. As multiple users can be scheduled within the same subframe, the UE specific search spaces associated to the connected users within the cell should not fully overlap. The common search spaces are used to send DCI messages intended to several or all users at the same time. Thus, a UE 102 is supposed to monitor its own configured UE specific search spaces for unicast transmissions as well as the common search spaces for primarily receiving for instance system information and paging, i.e. information that address all, or a group, of connected users. It can be noted that the common search spaces can also be used for unicast transmissions.
In connected mode, a UE 102 will be configured by higher layer signaling with two common search spaces and four UE specific search spaces. The common search spaces relate to aggregation levels of 4 and 8 CCEs, respectively, whereas the UE specific search spaces relate to one for each CCE aggregation level, i.e. 1, 2, 4 and 8 CCEs.
Heterogeneous networks with single cell-id prevent cell splitting of PDCCH resources within the coverage of the macro point. This implies that the PDCCH capacity is the same irrespective if low power points are introduced or not. As introducing heterogeneous networks are motivated to improve the mobile broadband user experience in cellular networks and at the same time meet a dramatically increasing number of mobile broadband users, there is an emerging need to enhance the PDCCH capacity. Furthermore, the requirement of transmitting cell specific reference symbols in all subframes prevents efficient solutions for saving energy in the base stations and therefore it is of interest to reduce the dependency of cell specific reference symbols in future LTE releases.
Relaying was introduced in LTE Rel. 10, see 3GPP TS 36.216 v10.1.0, “Physical layer for relaying operation”. Since relay nodes (RNs) might not be able to receive the regular control channel (PDCCH) from its donor eNB (DeNB) a new control channel, the R-PDCCH, (Relay PDCCH) was introduced. FIG. 5 illustrates a scheme 500 indicating an example of R-PDCCH transmission.
The R-PDCCH 502 is not transmitted in the L1/L2 control region 504, which is composed of the first (up to 4) OFDM symbols per subframe and which spans the entire frequency domain 306. Instead the R-PDCCH 502 is transmitted in the regular data region 506 of a subframe as illustrated in FIG. 5. In time domain 304, the R-PDCCH 502 starts at the 4th (in first slot 508 of a subframe) or first OFDM symbol of a slot (in second slot 510 of a subframe) and ends at the end of the slot. In frequency domain 306 it is transmitted on one or more resource blocks. Downlink assignments 512 are transmitted on an R-PDCCH 502 in the first slot 508 and uplink grants 514 are transmitted on an R-PDCCH 502 in the second slot 510. The R-PDCCH 502 can be transmitted on antenna ports with UE-specific reference symbols.
Similar to PDCCH transmissions, R-PDCCHs 502 can be transmitted by applying the concept of a search space, i.e. configuring candidate locations in the time-frequency grid where the receiver can expect an R-PDCCH transmission. For the R-PDCCH design two search spaces exist. The search space in the first slot 508 of a subframe contains candidate locations for downlink assignments 512 and the search space in the second slot 510 of a subframe contains candidate locations for uplink grants 514.
The R-PDCCH 502, or a similar control channel based on UE-specific reference signals, may be used to transmit control information to regular UEs 102 in future releases of LTE.
However, UEs 102 capable of receiving and detecting PDCCH and further control transmissions need to monitor search spaces associated with both ways of sending DCI messages. This implies that the number of blind detections needed by a UE 102 will increase significantly in comparisons with monitoring PDCCH only. This would significantly increase the requirements on processing capability of the receiver as well as the power consumption.